1. Field of the Invention
This invention relates generally to an apparatus and method for controlling vehicle creep in a hybrid electric vehicle (HEV).
2. Description of the Prior Art
A powershift transmission is a geared mechanism that includes no torque converter, but instead employs two input clutches driveably connected to an engine crankshaft. The transmission produces multiple gear ratios in forward and reverse drive and transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
In a vehicle powertrain that provides a continuous drive connection between a power source and the vehicle wheels, creep is the tendency for a vehicle, when at a stand-still or relatively low speed, to move slowly in either a forward or reverse direction, depending on the position of the gear selector, when neither the accelerator pedal nor brake pedal is depressed. Generally, a vehicle operator expects the vehicle to creep (1) from a standstill as the driver releases the brake pedal with no accelerator pedal input, and (2) from a coast down condition as the vehicle decelerates to a lower speed with little or no wheel brake or accelerator pedal input.
The vehicle will creep to a unique terminal speed, i.e., a relatively low speed, for a given road load and vehicle load. The creep speed declines with increasing road grade until it reaches zero and vehicle roll-back becomes imminent. The engine should never stall during a vehicle creep condition. The vehicle creep speed is a defined and specified vehicle requirement.
For a vehicle with a conventional automatic transmission, vehicle creep is automatically provided as a result of the torque transfer provided through the torque converter's fluid coupling. In a vehicle having a powershift transmission, the vehicle creep torque is provided by controlling the clutch torque capacity while slipping the clutch in order to prevent engine stall.
During a vehicle creep condition, the transmission clutch torque capacity is further increased as the driver releases the brake pedal. The increase in clutch torque capacity disturbs the control of the engine idle speed because the increase in clutch torque capacity loads the engine. Therefore, engine idle speed control must be coordinated with any increase in the clutch torque capacity in order to avoid poor engine idle speed control due to delayed engine torque response due to manifold filling as clutch torque capacity is increased, and potential engine stall if too much clutch torque capacity is provided while the engine torque has not increased accordingly.
Unlike a conventional vehicle powertrain having a powershift transmission, a hybrid electric vehicle with a powershift transmission includes multiple propulsion paths and “active” torque actuators, i.e., an engine and electric machines, which can be used during a vehicle creep condition. Therefore, a more sophisticated vehicle creep control system is needed to deal with the complexities and added powertrain operating modes of a HEV.